Conventionally, metal halide lamps have been employed for such uses as a light source for motor vehicle headlights and so forth. Conventional metal halide lamps typically have such a construction in which three types of materials, rare gas (gaseous matter), mercury (liquid matter), and halide of metal (solid matter), are enclosed in an arc tube. More specifically, an example of such lamps is as follows:
As shown in FIG. 12, an approximately spherical-shaped arc tube 101 encloses a fill material 102. The arc tube 101 is composed of a light-transmissive vessel made of quartz. Each of the ends of the arc tube 101 is sealed at a seal portion 103. A pair of tungsten electrodes 104 is provided in the arc tube 101. Each of the electrodes 104 is connected with a lead wire 106 via a molybdenum foil 105 hermetically sealed in the seal portion 103. The dimensions of this metal halide lamp are as follows:
Arc tube internal volume: 1.7 cc PA1 Distance between the electrodes 104: approx. 16 mm PA1 Hg (mercury): 21.5 mg (12.6 mg/cc) PA1 TlI (thallium iodide): 0.27 mg (0.16 mg/cc) PA1 InI (indium iodide): 0.04 mg (0.021 mg/cc) PA1 NaI (sodium iodide): 1.9 mg (1.14 mg/cc) PA1 Xe (xenon): 12 kPa (at room temperature) PA1 Arc tube internal volume: 0.025 cc PA1 Distance between electrodes: approx. 4 mm PA1 ScI.sub.3 (scandium iodide): 0.04 mg PA1 NaI (sodium iodide): 0.21 mg PA1 (The total weight of ScI.sub.3 and NaI is 0.25 mg) PA1 Xe (xenon): 10 atm (at room temperature) PA1 a rare gas, PA1 at least one of Sc (scandium) and halide of Sc, PA1 at least one of Na (sodium) and halide of Na, and PA1 at least one of metal and halide of the metal, PA1 the metal having an ionization potential as a simple substance of 5 to 10 eV, and the metal or halide of the metal having a vapor pressure of at least 10.sup.-5 atm at an operating temperature of the lamp. PA1 the halide of Sc is ScI3, PA1 the halide of Na is NaI, and PA1 the amount of Sc or halide of Sc, the amount of Na or halide of Na, the amount of the metal or halide of the metal, and a rated power of the lamp are determined so that the following equation is satisfied: EQU 1100.ltoreq.-4054+2759A+182C-1628A.sup.2 +18 AC-0.7C.sup.2 PA1 a rare gas, PA1 at least one of In (indium) and halide of In, PA1 at least one of TI (thallium) and halide of TI, and PA1 at least one of Na (Sodium) and halide of Na, the lamp in which: PA1 the amount of the In or halide of In is such an amount that an absorption spectrum is observed at approximately 410 nm and 451 nm in a spectral distribution, PA1 the amount of the Tl or halide of Tl is such an amount that an absorption spectrum is observed at approximately 535 nm in a spectral distribution, and PA1 the amount of the Na or halide of Na is such an amount that an absorption spectrum is observed at approximately 589 nm in a spectral distribution.
The contents in the fill material 107 are as follows:
When the lamp according to the above construction is operated under the condition where the electric current is controlled in order for the lamp power to be maintained at 100 W, a luminous flux of approximately 6200 lm is emitted by the electric discharge between the electrodes 104. In this operation, all of the mercury and a portion of the metal halides such as TlI etc. are evaporated, and a voltage (operating voltage) drop of 100 V is caused between the foremost ends of the electrodes 104.
The above-mentioned rare gas (Xe) is enclosed in order to facilitate a starting (start of discharge) and to increase the light output immediately after the starting. The metal halides (such as TlI) are enclosed in order to obtain an appropriate light output during a stable operation.
Mercury is enclosed in order to obtain a high voltage between the electrodes (operating voltage), which is required for the stable operation of the lamp. A voltage increasing effect of mercury is, more specifically, represented by the following equation as disclosed in, for example, Japanese Unexamined Patent Publication No. 06-13047 etc. EQU Vla=20+k (proportional constant).times.nHg.sup.0.56.times.L,
In the equation, Vla is an operating voltage (V), nHg is an amount of mercury per unit arc tube internal volume (mg/cc), and L is a distance between electrodes (mm).
From the equation, it is understood that the operating voltage is proportional to the product of the distance between electrodes and the approximately 1/2 power of an atomic density of mercury. In the above equation, the constant `20` is the sum of the voltage approximately at the electrodes and a voltage by the effects of the rare gas and metal halides. According to this equation, if mercury is not added, the operating voltage is greatly dropped (nHg=0, and thereby the operating voltage is approximately 20 V.). Therefore, the electric current is required to be increased in order to operate the lamp with the same power (in comparison with the case of the operating voltage being approximately 100 V, the required current is approximately SA, which is 5 times as large.). Hence, electrode losses are increased, and a conspicuous blackening of the arc tube is caused by a sputtered matter of the electrodes, thus deteriorating the luminous flux. Specifically, the arc tube is blackened in as short as several tens of hours and reaches the end of its lamp life.
In view of the above problem, in conventional lamps, the operating voltage is increased to be approximately 70 to 100 V by adjusting the amount of mercury, and thereby the lamp current is suppressed and the electrode losses (Joule loss) are also reduced. A long lamp life up to several thousand hours (for example, approximately 6000 hours) is thus achieved
However, while mercury brings about such a desirable effect that the operating voltage can be increased as above, it incurs such drawbacks as follows.
Firstly, since mercury causes a deterioration of luminous efficacy, attaining a bright lamp becomes difficult. That is because mercury has the second highest excitation potential in all the elements, next to rare gases, and therefore the light emission is little when compared with other metallic elements employed as metal halides. This fact is also seen from the spectral distribution of the above-described metal halide lamp, as shown in FIG. 13. Specifically, the emitted light of the lamp retains a plurality of line spectra, and the major wavelengths are 410.01 nm and 451.1 nm by In, 535.0 nm by Ti, and 589.0 nm and 589.6 nm by Na. Since mercury contributes little to the light emission, very little light emission by mercury is observed. On the other hand, in the case where no mercury is added in the above lamp, a high luminous efficacy of approximately 70 lm/W (the whole luminous flux is approximately 7000 lm) is obtained.
Secondly, a step of enclosing mercury, being a liquid matter, is necessary in the manufacturing steps of such a lamp, which tends to increase the manufacturing cost.
In addition to the above drawbacks, in recent years, metal halide lamps containing no mercury have been increasingly desired since a global environmental concern has been growing.
In view of these problems and perspectives, in order to raise an operating voltage without adding mercury, Japanese Unexamined Patent Publication No. 06-84496 etc. discloses an example of a technique in which the fill pressure of Xe is increased. More specifically, according to the description, in a metal halide lamp in which only a rare gas and metal iodides such as ScI.sub.3 and NaI are enclosed in the arc tube and no mercury is contained, an operating voltage of 50 V or higher can be achieved by satisfying the equation, EQU P.times.L.gtoreq.40,
where the distance between electrodes in the lamp is L (mm), and in the case of the rare gas to be enclosed being Xe, the fill pressure of Xe at room temperature is P (atm).
In accordance with the above teachings, the present applicants prepared a lamp that has the same shape as the one illustrated in the aforementioned FIG. 12, with the major dimensions and the fill material being as follows, and the operating voltage of the lamp was measured using the lamp thus prepared.
The fill material 107 contained the following.
In this lamp, P.times.L becomes 40, and therefore this lamp satisfies the condition of the above-described lamp. However, when this lamp was operated with a lamp power of 35 W, the operating voltage resulted in 35 V, falling short of 50 V described in the publication. As a result, electrode sputtering was caused by the large lamp current, which led to blackening of the arc tube wall by the sputtered electrode material attached to the arc tube inner wall, and consequently the emitted luminous flux was reduced in an early stage. It is considered from the above result, that, in order to obtain an operating voltage of 50 V or higher, the minimum Xe pressure (10 atm) which satisfies the condition of P.times.L.gtoreq.40 is insufficient, and according to the assumption made by the present inventors, it is necessary that the Xe pressure be controlled at a pressure of approximately 25 atm, which is far higher than 10 atm as set forth in the description.
However, controlling the fill pressure of Xe at such a high level incurs other drawbacks as described in the following.
Firstly, Xe shows a high ionization potential of approximately 12 eV, and therefore, in order to cause a discharge when the starting of the lamp at a pressure of over 25 atm, a considerably high starting voltage should be applied. More specifically, a lamp in which Xe is enclosed with a pressure of approximately 7 to 10 atm, a starting voltage required to ensure the start of discharge is 30 kV or above. On the other hand, in the case where the fill pressure is over 25 atm, a far higher starting voltage is required. Consequently, a complicated and large-scale starting circuit for generating the starting voltage becomes necessary, which incurs such disadvantages as an increase of the manufacturing cost and the like.
In addition, Xe has a relatively high excitation potential, and therefore, when Xe is enclosed with a high pressure, a decrease of luminous efficacy is induced.
Furthermore, in the case of the fill pressure being such a high pressure as above (note that this causes a further pressure increase in the arc tube in the lamp operation), there are increased possibilities of a burst of the arc tube and a leak of the fill material.
In summery, prior art metal halide lamps have such drawbacks that it is difficult to suppress the electric current by increasing the operating voltage of the lamp without adding mercury or making the inner pressure of the arc tube excessively high, and thereby to prolong the lamp life.
In addition, the prior art metal halide lamps that contain no mercury also have such a drawback that, since no light emission by mercury is obtained, the chromaticity deviation of the chromaticity coordinate of the emitted light by the lamps from a blackbody locus in a CIE 1960 u,v chromaticity diagram results in 0.011, and therefore, in cases of the uses for white light motor vehicle headlights, the lamps do not meet the standard of Gas Discharge Light Sources for Motor Vehicles Headlamps provided by the Japan Electrical Lamp Manufacturers Association (JEL 215).